Scanning backlight for flat-panel display

ABSTRACT

An illuminator for a flat-panel display comprises a tapered slab waveguide  1  co-extensive with the display, a light source  2 - 4  arranged to inject light into an edge of the waveguide so that it emerges over the face of the waveguide, and means for scanning the light injected into the wedge so that different areas of the panel are illuminated in turn. Preferably the light source is a set of rows of LEDs, each row injecting light at a different range of angles so that it emerges over different areas of the waveguide  1.

This invention describes a way of making displays, and is in particularintended to make possible a liquid-crystal display which can show movingimages without smear.

Liquid-crystal displays are more compact than cathode ray tubes and soare replacing them for television and for use in computer displays.However, televisions are often used to show sport and other fast-movingimages, and the images of moving objects such as balls and people getsmeared on liquid-crystal displays. This is not because the liquidcrystal switches slowly, but because the emission from a liquid-crystalpixel is sustained, whereas that from a cathode-ray tube is pulsed, aswill now be explained.

A liquid-crystal display conventionally comprises a liquid-crystal paneland a backlight. A picture is formed on the panel by spatial modulationof the transparency of the liquid crystal, and the picture is madevisible to the viewer by the backlight. The backlight must be thin, butfluorescent tubes that are thin yet large enough to illuminate aliquid-crystal panel are rather delicate. The backlight often thereforecomprises a thin transparent plastic wedge, and a cylindricalfluorescent tube adjacent to the thick end of the wedge—see for instanceEP-A1-663600 (Nitto Jushi).

Light emitted from the fluorescent tube over a range of directionsenters the wedge through its thick end, and then propagates along theaxis of taper by total internal reflection off the wedge/air interfaces,as shown in FIG. 1. Because the waveguide is tapered, i.e. its faces arenot quite parallel, each time a ray reflects off one side of the wedge,the ray's angle relative to the normal of the opposite side decreases.Eventually the critical angle is reached and the ray emerges into air.Unless they have been scattered, rays emerge from the wedge in adirection close to the plane of the wedge/air interface. A prismaticfilm is therefore often placed over the surface of the wedge in order todeflect the rays so that their average direction is perpendicular to thewedge/air interface.

The material out of which the wedge is made is often designed so that itslightly scatters light. The effect is that the direction of light whichemerges from the wedge is diffuse so that the image on theliquid-crystal panel can be seen over a wide field of view. One thusobtains a uniform illumination.

If an eye follows the image of a ball as it moves across a screen then,in order to avoid blur, the image of the ball should shift by one pixelevery time the centre of attention of the eye shifts by one pixel.However, the moving image on a video display is made of still picturescalled frames which are renewed at a set rate, namely every sixtieth ofa second. Suppose that the image of a moving ball is being displayed,and that the image shifts ten pixels between each frame. At the start ofeach frame, the image of the ball coincides instantaneously with thecentre of attention of the eye, but with each tenth of a frame periodthe centre of attention of the eye shifts by one pixel whereas the imageof the ball remains where it is. The image of the ball therefore becomesblurred by up to ten pixel widths.

Cathode ray tubes avoid blur because pixels are pulsed, so that the eyeonly sees the image of the ball at the start of each frame, or when thepixels are addressed, when the image coincides with the centre ofattention of the eye. The eye sees nothing further until the image againcoincides with the centre of attention of the eye, and the dark periodin between is imperceptible because the eye cannot detect flicker atperiods of a sixtieth of a second. One way of eliminating blur in aliquid-crystal display is to configure the liquid crystal so that itbehaves like a cathode ray tube by relaxing into a dark state soon afterbeing addressed. However, blocking light for large parts of the dutycycle wastes power. Another way of eliminating blur, as used in videoprojectors, is to scan illumination across the liquid-crystal display,but this requires bulk optic systems which are acceptable in videoprojectors but unacceptable within the flat form factor that makesliquid-crystal displays so attractive.

This invention aims to provide a flat-panel scanning illuminator whichcomprises a transparent wedge, and means for scanning the direction oflight injected into the thick end of the wedge.

According to the invention an illuminator for a flat-panel displaycomprises a slab waveguide of the tapered type co-extensive with thedisplay, a light source arranged to inject light into an edge of thewaveguide so that it emerges over the face of the waveguide, and meansfor scanning the light injected into the wedge so that different areasof the panel are illuminated in turn. Preferably the light source is aset of rows of LEDs, each row injecting light at a different range ofangles so that it emerges over different areas of the waveguide. Therows of LEDs are parallel to the input edge of the tapered waveguide.

In principle it would be possible to have any subdivision of the inputlight, though clearly it should be synchronised with the addressing ofthe display pixels. One could arrange two or more parallel strip lights,if they could be controlled at the necessary speed, or even a singlelight source with one or more shuttering or diverting means, whetherelectronic, optical or mechanical. In general the scanning of the inputlight would be in blocks corresponding to a number of rows on thedisplay, though in theory a subdivision along rows would not beimpossible.

The illuminator could be used as a light source for any kind of backlitdisplay, though liquid-crystal displays are ideally suited.

The waveguide may be literally tapered, i.e. so that it has across-section, in the direction of propagation through it beforeemergence at the face, that tapers down; or it may achieve the sameeffect by “optical tapering”, e.g. using variation in refractive index.

To aid understanding a specific embodiment of the invention will begiven by way of example, referring to the accompanying drawings, inwhich:

FIG. 1 shows how the angle at which light is injected into a wedgealters the position at which the light emerges;

FIG. 2 shows in side and front views how three lines of light-emittingdiodes can be placed at different positions within the focal plane of amirror so that each illuminates a different segment of theliquid-crystal display;

FIG. 3 is the same as FIG. 2 but shows how the screen is illuminated athird of a frame later;

FIG. 4 is the same as FIG. 2 but shows how the screen is illuminatedtwo-thirds of a frame later; and

FIG. 5 shows in side and front views how a pair of wedges with sharedthick ends can be used to direct light from three lines oflight-emitting diodes to different areas behind a liquid-crystal panel.

As shown in FIG. 2, a wedge-shaped, generally flat rectangular waveguide1, which is entirely transparent and free of scattering inclusions, hasits thick end, here at the bottom edge of the display, illuminated bylinear LED arrays 2, 3 and 4 which are in the focal plane of acylindrical mirror 5. Each array consists of a row of LEDs of diameterabout 5 mm, extending over the width of the display, which may beperhaps 30-100 cm.

The parallel light reflected off the mirror enters the thick end 10 ofthe wedge, which is bevelled so as to be roughly perpendicular to theincoming light, and bounces towards the thin end at ever shallowerangles until it escapes, at a position determined by its angle of input.The steeper the angle of input, the earlier the light escapes. Thisprinciple is described in EP-A1-663 600 (Nitto Jushi), for instance.However, here the input light is subdivided over the range of inputangles; the size of the LED arrays and the taper of the wedge are chosenso that the light from the LED (i.e. from a single row) escapes overonly a third of the height of the display. The three adjacent rowsbetween them thus cover the full height of the display. (Words such as“height”, “horizontal” and so forth are of course used with reference toa normally oriented display.)

This backlight assembly is used for a standard liquid-crystal display asfollows. Once the top third of a frame has been written to theliquid-crystal display, the first LED array 2 is illuminated. Light fromthis array is collimated into a set of angles which, after injectioninto the thick end of the wedge 1, go on to emerge from the top third ofthe wedge 1. Here the rays should be bent to the normal by a sheet ofprismatic film 6 and diffused, either before or after passing throughthe liquid-crystal display 7.

Once the centre third of the frame has been written to theliquid-crystal display, the first LED array 2 is switched off and thesecond LED array 3 is switched on, as shown in FIG. 3. Light from thesecond LED array 3 is collimated into an adjacent set of angles which,after injection into the thick end of the wedge, go on to emerge fromthe centre of the wedge.

Lastly, once the bottom third of the frame has been written to theliquid-crystal display, the middle LED array 3 is switched off and thethird LED array 4 is switched on so as to illuminate the bottom third ofthe liquid-crystal display. Simultaneously to this, the top third of thenext frame will begin to be written to the top third of theliquid-crystal panel, and so on. This is shown in FIG. 4.

The controller for the LEDs is synchronised with that of the display sothat each row 2, 3, 4 is illuminated when its corresponding third of thepanel is addressed, and it remains lit for a third of the cycle time.Clearly there could be two, four or in principle any number of rows. Fora standard 14-inch screen subdivision into three is found to give anacceptably low level of smear.

Colour pictures require red, green and blue LED's, and it isconventional to interleave these in order to mix light of differentcolours before it reaches the liquid-crystal display. Furthermore, it isdesirable generally to extend the thick end of the wedge beyond the baseof the liquid crystal display so that there is a length over whichmixing can take place, and so that the illumination is uniformly whiteat the base of the liquid-crystal panel and beyond. This extendedsection may need to be longer with the scanning-illumination schemebecause LED's illuminating a particular region of the liquid-crystalpanel at a particular wavelength will be more widely spaced than isconventional. If necessary, the extended section can be folded behindthe liquid-crystal panel with prisms, so the extra length of theextended section does not create an unacceptable change in form factor.

The cylindrical mirror is a slightly bulky element and it may beadvantageous to exchange it for a second short wedge 8 which acts as aninput element for the illumination wedge 1 as shown in FIG. 5. The inputwedge 8 acts in reverse, so that light injected near the tip of wedge 8from LED 2 forms rays with a shallow angle at the interface betweeninput wedge 8 and display wedge 1, and therefore emerges near the tip ofthe display wedge 1, whereas light injected near the base of the inputwedge 8 forms rays with a steep angle at the interface between thatwedge and the display wedge 1, so the light emerges near the base of thedisplay.

In the example described the scanning of the illumination is vertical.This is preferred because the scanning of the display is also vertical,i.e. row by row, but a division in the horizontal direction isconceivable. LEDs are also preferred as light sources because theyswitch on and off fast, but other sources could be used.

The example also uses geometrically tapered wedge-shaped waveguides, butthe same effect could be achieved with “optically tapered” waveguides,made for instance using GRIN techniques. For convenience such displaysare referred to as “tapered”.

1. A method of backlighting a display, the method comprising: turning ona light array of N light arrays in response to display data having beenwritten to a corresponding horizontal row of N horizontal rows of thedisplay, the turned-on light array configured for backlightingpredominately the corresponding horizontal row of the display; turningoff the turned-on light array in response to display data having beenwritten to another horizontal row of the N horizontal rows of thedisplay; and repeating the method in response to the turning off.
 2. Themethod of claim 1, the backlighting based on light emitted from theturned-on light array being injected into an input edge of a waveguidethat is configured for causing the injected light to emerge from thewaveguide predominately over the corresponding horizontal row of thedisplay.
 3. The method of claim 2 further comprising bending theemerging light to be substantially perpendicular to a viewing face ofthe display.
 4. The method of claim 2 further comprising diffusing theemerging light.
 5. The method of claim 2, each of the N light arraysconfigured for emitting light that is subdivided over a range of inputangles, and the input edge of the waveguide configured for causing thesubdivided light from the each of the N light arrays to emerge, based onthe range of input angles, from the waveguide predominately over acorresponding one of the N horizontal rows of the display.
 6. The methodof claim 5, the input edge of the waveguide substantially co-extensivewith a back of the display along a horizontal axis of the back of thedisplay.
 7. The method of claim 6, the waveguide disposed behind theback of the display linearly tapered along a vertical axis of the backof the display.
 8. The method of claim 1, each of the N light arrayscomprising a row of light emitting diodes.
 9. The method of claim 1,each of the N light arrays configured for backlighting predominately adistinct corresponding one of the N horizontal rows of the display. 10.The method of claim 1, each of the N light arrays comprising a row ofinterleaved red, green, and blue light emitting diodes.
 11. Anilluminator system for a display, the illuminator system comprising: acontroller configured for turning on a light array of N light arrays inresponse to display data having been written to a correspondinghorizontal row of N horizontal rows of the display, the turned-on lightarray configured for backlighting predominately the correspondinghorizontal row of the display; the controller further configured forturning off the turned-on light array in response to display data havingbeen written to another horizontal row of the N horizontal rows of thedisplay; and the controller further configured for repeating the turningon and then the turning off in response to the turning off.
 12. Themethod of claim 11, the backlighting based on light emitted from theturned-on light array being injected into an input edge of a waveguidethat is configured for causing the injected light to emerge from thewaveguide predominately over the corresponding horizontal row of thedisplay.
 13. The method of claim 12 further comprising a film configuredfor bending the emerging light to be substantially perpendicular to aviewing face of the display.
 14. The method of claim 12, furthercomprising the input edge configured for scattering the emitted light sothat the emerging light is diffuse.
 15. The method of claim 12, each ofthe N light arrays configured for emitting light that is subdivided overa range of input angles, and the input edge of the waveguide configuredfor causing the subdivided light from the each of the N light arrays toemerge, based on the range of input angles, from the waveguidepredominately over a corresponding one of the N horizontal rows of thedisplay.
 16. The method of claim 15, the input edge of the waveguidesubstantially co-extensive with a back of the display along horizontalaxis of the back of the display.
 17. The method of claim 16, thewaveguide disposed behind the back of the display and linearly taperedalong a vertical axis of the back face of the display.
 18. The method ofclaim 11, each of the N light arrays comprising a row of light emittingdiodes.
 19. The method of claim 11, each of the N light arraysconfigured for backlighting predominately a distinct corresponding oneof the N horizontal rows of the display.
 20. The method of claim 11,each of the N light arrays comprising a row of interleaved red, green,and blue light emitting diodes.